Crack initiation, propagation and strain mapping during composite bending using digital image correlation and theoretical calculations

The study focuses on analyzing strains and stresses in bent specimens of various dimensions. Depending on the maximum normal to shear stress ratio during bending, two failure modes can occur: translaminar and interlaminar. A higher value of this ratio increases the likelihood of translaminar failure. The research demonstrates that the critical value for polypropylene reinforced with unidirectional continuous glass fibers is 24. In this case, specimens exhibited both translaminar and interlaminar failure modes. The study also examined the specimens’ appearance at different stages of the bending test, as well as their microstructure after failure. Digital image correlation (DIC) was employed, enabling the determination of strain values for individual specimens and the comparison of theoretical values with those obtained experimentally via DIC. It was found that while the linear strain along the fibers and the shear strain were consistent, the actual strain across the specimen’s height was significantly higher than the theoretically calculated value. This discrepancy is likely due to the occurrence of additional stresses within the specimen, particularly associated with Hertzian contact phenomena, which caused localized compression and further influenced the strain distribution.